Sleep apnea, the temporary cessation of respiratory airflow during sleep, is generally considered a medical syndrome that occurs in at least three recognized forms. The first is central sleep apnea, associated with the failure of the central nervous system to automatically initiate and control respiration. The second is obstructive sleep apnea, associated with an obstruction of the airways due to their collapse. A third, mixed form may include a central nervous system failure to drive ventilatory effort combined with an obstructive apnea. Daytime sleepiness and associated cardiovascular diseases significantly impair patient lifestyle and increase morbidity risk. Various approaches have been taken to detect and treat sleep apnea.
A standard diagnostic approach for sleep apnea includes polysomnography, which requires the patient to stay overnight in a hospital for observation, in addition to medical history and screening questionnaires. Polysomnography involves monitoring of multiple parameters including electroencephalography, electromyography, electrocardiography, oximetry, airflow, respiratory effort, snoring, body position and blood pressure. This intensive and costly approach is not practical for screening large numbers of patients, yet the prevalence of undiagnosed sleep apnea in the U.S. is thought to be in the millions with on the order of 2% of middle-aged women and 4% of middle-aged men having sleep apnea syndrome. See Young T. et al., “The occurrence of sleep-disordered breathing among middle-aged adults,” New England J. Med. 1993;328:1230-1235. An apnea-hypopnea index (AHI) is used by physicians to gauge the severity of sleep apnea. AHI is the number of apnea-hypopnea episodes occurring per hour during a monitored period. It is estimated that 9% of women and 24% of men have an AHI greater than 5, indicating at least a mild to moderate sleep apnea condition.
A method for screening and diagnosing sleep apnea that is less costly and less stressful to the patient than polysomnography is needed, therefore, in order to reach the large number of patients having undiagnosed sleep apnea. A method of identifying a breathing parameter that is characteristic of the breathing status of a sleeping individual and measuring the derivative trend with respect to time of one variable of state of the cardiovascular system, which variable recurrently changes with the respiration is generally disclosed in U.S. Pat. Application No. 20020169384 to Kowallik et al. A microprocessor system for the simplified diagnosis of sleep apnea, which includes an inexpensive system for the collection and analysis of pulse oximetry values as a function of time during sleep is generally disclosed in U.S. Pat. No. 20020173707 to Lynn et al.
Once diagnosed, a common mode of treatment is application of continuous positive airway pressure (CPAP) to maintain patency of the airways. Continuous positive airway pressure is applied throughout the night and can cause considerable stress to the patient. Alternative therapeutic approaches involve detecting the onset of an apnea episode and then delivering a therapy to either maintain airway patency or counteract autonomic-mediated causes during apnea. For example, electrical stimulation of the hypoglossal nerve or muscles of the upper airways has been proposed or attempted clinically. Reference is made to U.S. Pat. No. 5,540,733 issued to Testerman et al., U.S. Pat. No. 5,174,287 issued to Kallok, and U.S. Pat. No. 6,251,126 issued to Ottenhoff et al., all of which patents are incorporated herein by reference in their entirety.
Detection of sleep apnea for the purposes of triggering the delivery of a sleep apnea therapy may be based on respiratory monitoring. Measuring respiratory effort by monitoring airway pressures is generally disclosed in the above-cited U.S. Pat. No. 5,540,733 issued to Testerman and in U.S. Pat. No. 6,132,384 issued to Christopherson et al. A method for monitoring electrical activity associated with contractions of the diaphragm and the pressure within the thorax and upper airway is generally disclosed in U.S. Pat. No. 5,174,287 issued to Kallok.
Sleep apnea is known to have cardiovascular consequences including changes in cardiac rhythm, hemodynamic fluctuations, and hypertension. Low oxygen levels due to sleep apnea are associated with an increased morbidity due to cardiovascular complications, including heart attack and stroke. Overdrive pacing of the heart upon detection of sleep apnea is proposed in U.S. Pat. No. 6,126,611 issued to Bourgeois et al., incorporated herein by reference in its entirety. However, a high pacing rate may tend to arouse the patient, ending the apnea. Although, the inventors hereof believe that significantly increased nocturnal overdrive pacing (NOP)—relative to a typical mean pacing rate during sleep will not wake a patient.
Heart rate variability and frequency and time domain analysis of the heart rate have also been proposed for detecting apnea. Frequency and time domain analysis of heart rate variability may require sophisticated algorithms that may be performed on a personal computer but may not be suitable for implementation in an implantable device because of the intensive microprocessing time and power required. Cyclical variation of the heart rate in sleep apnea syndrome has been observed as progressive bradycardia, followed by abrupt tachycardia on resumption of breathing. See Guilleminault C. et al., “Cyclical variation of the heart rate in sleep apnoea syndrome: Mechansims and usefulness of 24 h electrocardiography as a screening technique.” QT interval changes have also been observed during obstructive sleep apnea syndrome with a progressive prolongation of the QT interval during apnea and an abrupt shortening during the postapnea period. Significant changes in the R-R interval or QT interval were not observed during normal REM sleep. Ambulatory ECG monitoring may be used for monitoring for these electrocardiogram changes, however, external monitoring or therapy delivery devices are generally subject to the limitation of patient compliance.
An improved method is needed for detecting sleep apnea or other disordered breathing patterns that does not require complicated sensors or signal processing and may be readily implemented in an implantable device. The method preferably provides diagnostic or prognostic data that may be used for screening, diagnosing, and monitoring patients for apnea or other breathing disorders such as Cheyne-Stokes breathing and may provide apnea detection for triggering the delivery of a therapy and monitoring for therapy evaluation.